A   BRIEF  ACCOUNT  OF 
RADIO-ACTIVITY 


BY 

FRANCIS  P.  VENABLE,  PH.D.,  D.Sc.,  LL.D. 

PROFESSOR  OF  CHEMISTRY,   UNIVERSITY  OF  NORTH  CAROLINA 

AUTHOR  OF 
"A  SHORT  HISTORY  OF  CHEMISTRY," 

"PERIODIC  LAW,"  ETC. 


D.  C.  HEATH  &  CO.,   PUBLISHERS 

BOSTON  NEW   YORK  CHICAGO 


V4- 


COPYRIGHT,  1917, 
BY  D.  C.  HEATH  &  Co. 

IA7 


PREFACE 

I  HAVE  gathered  the  material  for  this  little  book  because 
I  have  found  it  a  necessary  filling  out  of  the  course  for  my 
class  in  general  chemistry.  Such  a  course  dealing  with  the 
composition  and  structure  of  matter  is  left  unfinished  and 
in  the  air,  as  it  were,  unless  the  marvellous  facts  and  deduc- 
tions from  the  study  of  radio-activity  are  presented  and 
discussed.  The  usual  page  or  two  given  in  the  present 
text-books  are  too  condensed  in  their  treatment  to  afford 
any  intelligent  grasp  of  the  subject,  so  I  have  put  in  book 
form  the  lectures  which  I  have  hitherto  felt  forced  to  give. 

Perhaps  the  book  may  prove  useful  also  to  busy  men  in 
other  branches  of  science  who  wish  to  know  something  of 
radio-activity  and  have  scant  leisure  in  which  to  read  the 
larger  treatises. 

It  is  needless  to  say  that  there  is  nothing  original  in  the 
book  unless  it  be  in  part  the  grouping  of  facts  and  order  of 
their  treatment.  I  have  made  free  use  of  the  writings  of 
Rutherford,  Soddy,  and  J.  J.  Thomson,  and  would  here 
express  my  debt  to  them  —  just  a  part  of  that  indebtedness 
which  we  all  feel  to  these  masters.  I  wish  also  to  acknowl- 
edge my  obligations  to  Professor  Bertram  B.  Boltwood  for 
his  helpful  suggestions  in  connection  with  this  work. 


m 


37169G 


CONTENTS 

CHAPTER  I 

DISCOVERY  OF  RADIO-ACTIVITY  PAGE 

The     beginning  —  Radio-active    bodies  —  An    atomic    property  —    , 
Discovery  of  new  radio-active  bodies  —  Discovery  of  Polonium 
—  Discovery  of  Radium  —  Other  radio-active  bodies  found     .    .         1 

CHAPTER  II 

PROPERTIES  OF  THE  RADIATIONS 

lonization  of  Gases  —  Experimental  confirmation  —  Application  of 
electric  field  —  Size  and  nature  of  ions  —  Photographing  the 
track  of  the  ray  —  Action  of  radiations  on  photographic  plates  — 
Discharge  of  electrified  bodies  —  Scintillations  on  phosphorescent 
bodies  —  Penetrating  power  —  Magnetic  deflection  —  Three 
types  of  rays — Alpha  rays  —  Beta  rays  —  Gamma  rays  — 
Measurement  of  radiations  —  Identifications  of  the  rays  ...  7 

CHAPTER  III 
CHANGES  IN  RADIO-ACTIVE  BODIES 

Radio-activity  a  permanent  property  —  Induced  activity  —  Discovery 
of  Uranium  X — Conclusions  drawn — Search  for  new  radio- 
active bodies  —  Methods  of  investigation  —  Nature  of  the 
radiations — Life-periods  —  Equilibrium  series 17 

CHAPTER  IV 
NATURE  OF  THE  ALPHA  PARTICLE 

Disintegrating  of  the  elements  —  Identification  of  the  rays  —  The 
alpha  rays  —  Alpha  rays  consist  of  solid  particles  —  Electrical 
charge — Helium  formed  from  alpha  particles  —  Discovery  of 
Helium  —  Characteristics  of  Helium  —  Table  of  constants  ...  25 


vi  CONTENTS 

CHAPTER  V 

THE  STRUCTURE  OF  THE  ATOM 

Properties  of  Radium  —  Energy  evolved  by  radium  —  Necessity  for  a 
disintegration  theory  —  Disintegration  theory  —  Constitution 
of  the  atom  —  Rutherford's  atom  —  Scattering  of  alpha  particles 

—  Stopping  power  of  substances 32 

CHAPTER  VI 

RADIO-ACTIVITY  AND  CHEMICAL  THEORY 

Influence  upon  chemical  theory  —  The  periodic  system  —  Basis  of  the 
periodic  system  —  Influence  of  positive  nucleus  —  Determination 
of  the  atomic  number  —  Use  of  X-ray  spectra  —  Changes  caused 
by  ray-emission  —  Atomic  weight  losses  —  Lead  the  end  product 

—  Changes  of  position  in  the  periodic  system  —  Changes  from 
loss  of  beta  particles  —  Isotopes  —  Radio-activity  in  nature  — 
Radio-active  products  in  the  earth's  crust  —  Presence  in  air  and 

soil  waters  —  Cosmical  radio-activity 41 

INDEX  .  ....  53 


A  BRIEF  ACCOUNT  OF 
RADIO-ACTIVITY 


A  BEIEF  ACCOUNT  OF 
EADIO-ACTIVITY 

CHAPTER  I 
DISCOVERY  OF  RADIO-ACTIVITY 

THE  object  of  this  brief  treatise  is  to  give  a  simple 
account  of  the  development  of  our  knowledge  of  radio- 
activity and  its  bearing  on  chemical  and  physical 
science.  Mathematical  processes  will  be  omitted,  as 
it  is  sufficient  to  give  the  assured  results  from  calcu- 
lations which  are  likely  to  be  beyond  the  training  of 
the  reader.  Experimental  evidence  will  be  given  in 
detail  wherever  it  is  fundamental  and  necessary  to  a 
confident  grasp  of  some  of  the  marvelous  deductions 
in  this  new  branch  of  science.  Theories  cannot  be 
avoided,  but  the  facts  remain  while  theories  grow  old 
and  are  discarded  for  others  more  in  accord  with  the 
facts. 

As  so  often  happens  in  the  history  of  science,  the 
opening  up  of  this  new  field  with  its  fascinating  dis- 
The  closures  was  due  to  an  investigation  under- 

Beginning  taken  for  another  purpose  but  painstakingly 
carried  out  with  a  mind  open  to  the  truth  wherever  it 
might  lead. 


2  RADIOACTIVITY 

In  1895,  Rontgen  modestly  announced  his  discovery 
of  the  X  rays.  This  attracted  immediate  and  intense 
interest.  Among  those  who  undertook  to  follow  up 
these  phenomena  was  Becquerel,  who,  because  of  the 
apparent  connection  with  phosphorescence,  tried  the 
action  of  a  number  of  phosphorescent  substances  upon 
the  photographic  plate,  the  most  striking  characteristic 
of  the  X  rays  being  their  effect  upon  such  sensitive 
plates.  In  these  experiments  he  obtained  no  results 
until  he  tried  salts  of  uranium,  recalling  previous 
observations  of  his  as  to  their  phosphorescence.  Dis- 
tinct action  was  noted.  Furthermore,  he  proved  that 
this  had  no  connection  with  the  phenomenon  of  phos- 
phorescence, as  both  uranic  and  uranous  salts  were 
active  and  the  latter  show  no  phosphorescence.  Bec- 
querel announced  his  discoveries  in  1896  and  this  was 
the  beginning  of  the  new  science  of  radio-activity. 

The  rays  given  off  by  uranium  and  its  salts  were 
found  to  differ  from  the  X  rays.  They  showed  no 
Radio-active  appreciable  variation  in  intensity,  no  pre- 
Bodies  vious  exposure  of  the  substance  to  light 

was  necessary,  and  neither  changes  of  temperature 
nor  any  other  physical  or  chemical  agency  affected 
them. 

At  first  uranium  and  its  compounds  were  the  only 
known  source  of  these  new  radiations,  but  many  other 
substances  were  examined  and  two  years  later  thorium 
and  its  compounds  were  added  to  the  list.  In  general 
the  discharging  action  seemed  about  the  same.  Other 


DISCOVERY  3 

elements  and  ordinary  substances  show  a  minute 
activity.  Only  potassium  and  rubidium  have  a  greater 
activity  than  this,  and  theirs  is  only  about  one- 
thousandth  that  of  uranium. 

In  the  examination  of  uranium  and  thorium  com- 
pounds it  was  found  that  the  activity  was  determined 
An  Atomic  by  the  uranium  and  thorium  present;  it 
Property  was  proportional  to  the  amount  of  these 
elements  present  and  independent  of  the  nature  of  the 
other  elements  composing  the  compound.  The  con- 
clusion was,  therefore,  that  the  activity  was  an  inherent 
property  of  the  atoms  of  uranium  and  thorium,  that  is, 
an  atomic  property.  This  was  a  long  step  forward  and 
introduced  into  science  the  conception  of  a  new  prop- 
erty of  matter,  or  at  least  of  certain  forms  of  matter. 
In  examining  a  large  number  of  minerals  contain- 
ing uranium  and  thorium,  Mme.  Curie  made  the 
Discovery  of  important  observation  that  many  of  these 
New  Radio-  were  more  active  than  the  elements  them- 
ies  selves.  In  measuring  the  activity  she 
made  use  of  the  electrical  method  which  will  be  de- 
scribed later.  In  the  following  table  giving  her  results 
for  uranium  minerals  the  numbers  under  i  give  the 
maximum  current  in  amperes.  They  serve  simply  for 
comparison. 

t 

Pitchblende  from  Joachimsthal 7.0  X  10  ~u 

Clevite 1.4  X  1Q-11 

Chalcolite    5.2  X  1Q-11 

Autunite    .  .  .2.7x10  ~u 


4  RADIO-ACTIVITY 

• 

Carnotite 6.2  x  1Q-11 

Uranium 2.3  x  10  ~n 

Uranium  and  potassium  sulphate . . .  0.7  X  10  ~u 
Uranium  and  copper  phosphate 0.9  X  10  ~u 

The  last  three  are  pure  uranium  and  compounds  of 
that  element  given  for  comparison  with  the  first  five, 
which  are  naturally  occurring  minerals.  The  last  com- 
pound has  the  same  composition  as  chalcolite  and  is 
simply  the  artificially  prepared  mineral.  It  has  the 
activity  which  would  be  calculated  from  the  propor- 
tion of  uranium  present,  the  copper  and  phosphoric 
acid  contributing  no  activity. 

Since  the  activity  is  not  dependent  upon  the  composi- 
tion but  upon  the  amount  of  uranium  present,  the  activ- 
ity in  all  of  the  minerals  should  be  less  than  that  of 
uranium.  On  the  contrary,  it  is  several  times  greater. 
Natural  and  artificial  chalcolite  also  show  a  marked 
difference  in  favor  of  the  former.  The  supposition 
was  a  natural  one,  therefore,  that  these  minerals  con- 
tained small  quantities  of  an  element,  or  elements, 
undetected  by  ordinary  analysis  and  having  a  much 
greater  activity  than  uranium.  Similar  results  were 
obtained  in  the  examination  of  thorium  minerals  and 
thorium  salts. 

Following  up  this  supposition,  M.  and  Mme.  Curie 
set  themselves  the  task  of  separating  this  unknown  sub- 
Discove-y  of  stance.  Starting  with  pitchblende,  a  sys- 
Poionium  tematic  chemical  examination  was  made. 
This  is  an  exceedingly  complex  mineral,  containing 


DISCOVERY  5 

many  elements.  The  processes  were  laborious  and 
demanded  much  time  and  minute  care.  They  need 
not  be  described  here.  It  is  sufficient  to  say  that  along 
with  bismuth  a  very  active  substance  was  separated,  to 
which  Mme.  Curie  gave  the  name  of  polonium  for 
Poland,  her  native  land.  Its  complete  isolation  is 
very  difficult  and  sufficient  quantities  of  the  pure  sub- 
stance have  not  been  obtained  to  determine  its  atomic 
weight  and  other  properties,  but  some  of  the  lines  of 
its  spectrum  have  been  determined.  Chemically  it 
is  very  closely  analogous  to  bismuth. 

In  a  similar  manner  a  barium  precipitate  was  ob- 
tained from  pitchblende  which  contained  a  highly 
Discovery  of  active  substance.  The  pure  chloride  of  this 
Radium  body  and  barium  can  be  prepared  together 

and  then  separated  by  fractional  crystallization.  To 
the  new  body  thus  found  the  name  of  radium  was 
given.  It  is  similar  in  chemical  properties  to  barium. 
Its  atomic  weight  has  been  determined  by  several  care- 
ful investigators  and  is  accepted  as  226.  Its  spectrum 
has  been  mapped  and  its  general  properties  are  known. 
It  is  a  silvery  white,  oxidizable  metal.  In  one  ton  of 
pitchblende  about  0.2  gram  of  radium  is  present;  this 
is  about  5000  times  greater  than  the  amount  of  polonium 
present.  The  activity  of  the  products  was  depended 
upon  as  the  guide  in  these  separations.  The  radium 
found  is  relatively  enormously  more  active  than  the 
pitchblende  or  uranium.  • 

In  the  above  separations  use  was  made  of  relation- 


6  RADIO-ACTIVITY 

ships  to  bismuth  and  barium.  Similarly,  by  taking 
other  Radio-  advantage  of  chemical  relationship  to  the 
active  Bodies  iron  group  of  elements,  another  body  was 
partially  separated  by  Debierne,  to  which 
he  gave  the  name  actinium.  Bolt  wood  discovered  in 
uranium  minerals  the  presence  of  a  body  which  he 
named  ionium,  and  which  is  so  similar  to  thorium 
that  it  cannot  be  separated  from  it.  It,  however,  far 
exceeds  thorium  in  activity. 

The  lead  which  is  present  in  uranium  and  thorium 
minerals  —  apparently  in  fairly  definite  ratio  to  the 
amount  of  uranium  and  thorium  —  is  found,  on  separa- 
tion and  purification,  to  possess  radio-active  properties. 
This  activity  is  due  to  the  presence  of  a  very  small 
proportion  of  an  active  constituent  called  radio-lead, 
which  has  chemical  properties  identical  with  those  of 
ordinary  lead.  The  bulk  of  the  lead  obtained  from 
radio-active  minerals  differs  in  atomic  weight  from 
ordinary  lead  and  appears  also  to  be  different  according 
to  whether  its  source  is  a  thorium  or  a  uranium  mineral. 

A  large  number  of  other  radio-active  substances  have 
been  separated  and  some  of  their  properties  determined, 
but  these  were  found  by  different  means  and  will  be 
noted  in  their  proper  place.  They  number  in  all  more 
than  thirty.  The  sources  or  parents  of  these  are  the 
original  uranium  or  thorium,  and  the  products  form 
regular  series  with  distinctive  properties  for  each 
member. 


CHAPTER  II 

PROPERTIES   OF   THE   RADIATIONS 

THE  activity  of  these  radio-active  bodies  consists  in 
the  emission  of  certain  radiations  which  may  be  sepa- 
rated into  rays  and  studied  through  the  phenomena 
which  they  cause. 

One  of  these  phenomena  is  the  power  of  forming  ions 
or  carriers  of  electricity  by  the  passage  of  the  rays 
lonization  through  a  gas,  thus  ionizing  the  gas.  The 
of  Gases  details  of  an  experiment  will  serve  to  make 
the  meaning  of  this  ionization  clear. 


t 


v 

•+V 
FIG.  1.  —  IONIZATION  OF  GASES. 

When  this  apparatus  is  set  up  a  minute  current  will 
be  observed  without  the  introduction  of  any  radio- 
active matter.  This,  as  Rutherford  says,  has  been 
found  due  mainly  to  a  slight  natural  radio-activity  of 
the  matter  composing  the  plates.  If  radio-active  mat- 


8  RADIO-ACTIVITY 

ter  is  spread  on  plate  A,  which  is  connected  with  one 
pole  of  a  grounded  battery,  and  if  plate  B  is  connected 
with  an  electrometer  which  is  also  connected  with  the 
earth,  a  current  is  caused  which  increases  rapidly  with 
the  difference  of  potential  between  the  plates,  then 
more  slowly  until  a  value  is  reached  that  changes  only 
slightly  with  a  larger  increase  in  the  voltage. 

According  to  the  theory  of  ionization,  the  radiation 
produces  ions  at  a  constant  rate.  The  ions  carrying  a 
positive  charge  are  attracted  to  plate  B,  while  those 
negatively  charged  are  attracted  to  plate  A,  thus  caus- 
ing a  current.  These  ions  will  recombine  and  neutralize 
their  charges  if  the  opportunity  is  given.  The  num- 
ber, therefore,  increases  to  a  point  at  which  the  ions 
produced  balance  the  number  recombining. 

When  an  electric  field  is  produced  between  the  plates, 
the  velocity  of  the  ions  between  the  plates  is  increased 
in  proportion  to  the  strength  of  the  electric  field.  In  a 
weak  field  the  ions  travel  so  slowly  that  most  of  them 
recombine  on  the  way  and  consequently  the  observed 
current  is  very  small.  On  increasing  the  voltage  the 
speed  of  the  ions  is  increased,  fewer  recombine,  the 
current  increases,  and,  when  the  condition  for  recom- 
bination is  practically  removed,  it  will  have  a  maximum 
value.  This  maximum  current  is  called  the  saturation 
current  and  the  value  of  the  potential  difference  required 
to  give  this  maximum  current  is  called  the  saturation 
P.D.  or  saturation  voltage. 

The  picture,  then,  is  this.     The  radiations  separate 


PROPERTIES   OF  THE   RADIATIONS  9 

the  components  of  the  gas  into  ions,  or  carriers  of  elec- 
tricity, half  of  which  are  charged  negatively  and  half 
positively.  In  the  electric  field  those  negatively 
charged  seek  the  positive  plate  and  those  positively 
charged  seek  the  negative  plate.  If  time  is  given,  these 
ions  meet  and  recombine,  their  charges  are  neutralized, 
and  there  is  no  current. 

This  theory  of  the  ionization  of  gases  has  been  most 
interestingly  confirmed  by  direct  experiment.  For 
Experimental  instance,  the  ions  may  form  nuclei  for  the 
Confirmation  condensation  of  water,  and  in  this  way  the 
existence  of  the  separate  ions  in  the  gas  may  be  shown 
and  the  number  present  actually  counted. 

When  air  saturated  with  water  vapor  is  allowed  to 
expand  suddenly,  the  water  present  forms  a  mist  of 
small  globules.  There  are  always  small  dust  particles 
in  air  and  around  these  as  nuclei  the  drops  are  formed. 
These  drops  will  settle  and  thus  by  repeated  small 
expansions  all  dust  nuclei  may  be  removed  and  no 
mist  or  cloud  will  be  formed  by  further  expansions. 

If  now  the  radiation  from  a  radio-active  body  be 
introduced  into  the  condensation  vessel,  a  new  cloud 
is  produced  in  which  the  water  drops  are  finer  and  more 
numerous  according  to  the  intensity  of  the  rays.  On 
passing  a  strong  beam  of  light  through  the  condensa- 
tion chamber,  the  drops  can  readily  be  seen.  These 
drops  form  on  the  ions  produced  by  the  radiation. 

If  the  condensation  chamber  has  two  parallel  plates 
for  the  application  of  an  electric  field  like  that  already 


10  RADIO-ACTIVITY 

described,  the  ions  will  be  carried  at  once  to  the  elec- 
trodes and  disappear.  The  rapidity  of  this  action 
depends  upon  the  strength  of  the  electric  field  and 
Application  of  experiment  shows  that  the  stronger  the  field 
Electric  Field  foe  smaller  the  number  of  condensation 
drops  formed.  If  there  is  no  electric  field,  a  cloud  can 
be  produced  some  time  after  the  shutting  off  of  the 
source  of  radiation,  showing  that  time  is  required  for  the 
recombination  of  the  ions. 

If  the  drops  are  counted  (there  being  special  methods 
for  this)  and  the  total  current  carried  accurately 
Size  and  Na-  measured,  then  the  charge  carried  by  each 
ture  of  ions  ;on  may  fa  calculated.  This  has  been 
determined.  The  mass  of  an  ion  compared  with  the 
mass  of  the  molecules  of  gas  in  which  it  was  produced 
can  also  be  approximately  estimated.  In  the  study 
of  these  ions  the  view  has  been  held  that  the  charged 
ion  attracted  to  itself  a  cluster  of  molecules  which  sur- 
rounded the  charged  nucleus  and  traveled  with  it.  It 
is  roughly  estimated  that  about  thirty  molecules  of 
the  gas  cluster  around  each  charged  ion. 

Utilizing  the  fact  that  these  ions  with  their  clusters 
of  molecules  form  nuclei  for  the  condensation  of  water 
Photo  a  h  vapor,  C.  T.  R.  Wilson  has  by  instantaneous 
ing  the  Track  photography  been  able  to  photograph  the 
track  of  an  ionizing  ray  through  air.  The 
number  of  the  ions  produced,  and  hence  the  number  of 
drops,  is  so  great  that  the  trail  is  shown  as  a  continuous 
line.  u.  In  the  copy  of  this  photograph  it  will  be  seen 


PROPERTIES   OF  THE   RADIATIONS         11 

that  at  some  distance  from  its  source  the  straight  trail 
is  slightly  but  abruptly  bent.  Near  the  end  of  its 
course  there  is  another  abrupt  and 
much  sharper  bend.  These  bends 
show  where  the  ionizing  ray,  in  this 
case  an  alpha  particle,  has  been  de- 
flected by  more  or  less  direct  collis- 
ion with  an  atom.  These  collisions 
and  the  final  disappearance  of  the 
ray  will  be  discussed  later. 

Taking  up  now  other  means  of  ex- 
amining these  radiations,  it  is  well  to 
consider  their  action  upon 

Action  of  Ra-  •.  •,  .  . 

diations  on      a  photographic  or  sensi- 
tive  plate.    It  will  be  re- 
called   that   this   was    the  FIG.  2.  —  PHOTOGRAPH 
method  by  which  their  existence  was      °F  ™  J^*  OF  AN 
originally  detected.    To  illustrate  the 
method,  the  following  account  of  how  one  such  photo- 
graph was  taken  may  be  given. 

The  plate  was  wrapped  in  two  thicknesses  of  black 
paper.  The  objects  were  placed  upon  this  and  the 
radio-active  ore,  separated  by  a  board  one  inch  thick, 
was  placed  above.  The  exposure  lasted  five  days.  The 
action  is  much  less  rapid  and  the  result  not  so  clearly 
defined  as  in  the  case  of  photographs  taken  by  X  rays. 
Of  course,  the  removal  of  the  board  and  the  use  of 
more  concentrated  preparations  of  radium  would  give 
quicker  and  better  results.  The  method,  however,  on 


12  RADIO-ACTIVITY 

account  of  time  consumed  and  lack  of  definition  is  ill 
adapted  to  accurate  work. 

The  radiations  from  radio-active  bodies  can  discharge 
both  positively  and  negatively  electrified  bodies  by 
Discharge  of  ma^mg  tne  ^  surrounding  them  a  con- 
Electrified  ductor  of  electricity.  To  demonstrate  this, 
use  is  made  of  an  electroscope.  If  the 
hinged  leaf  of  such  an  instrument  be  electrically  charged 
and  a  radio-active  body  be  brought  into  its  neighbor- 
hood, the  electricity  will  be  discharged  and  the  leaf 
return  to  its  original  position.  The  rapidity  of  this 
discharge  is  used  to  measure  the  degree  of  activity  of 
the  body  giving  off  the  radiation. 


FIG.  4.  —  GOLD-LEAF  ELECTROSCOPE. 

The  gold-leaf  L  is  attached  to  a  flat  rod  R  and  is  insulated  inside  the 
vessel  by  a  piece  of  amber  S  supported  from  the  rod  P.  The  system  is 
charged  by  a  bent  rod  CC'  passing  through  an  ebonite  stopper.  After 
charging,  it  is  removed  from  contact  with  the  gold-leaf  system.  The  rods 
P  and  C  and  the  cylinder  are  then  connected  with  the  earth. 


I 


\ 


FIG.  3  —  PHOTOGRAPH  OF  VARIOUS  OBJECTS 

TAKEN   BY   MEANS   OF   PITCHBLENDE 


PROPERTIES   OF  THE   RADIATIONS         13 

It  was  found  by  Crookes  that  a  screen  covered  with 

phosphorescent  zinc  sulphide  was  brightly  lighted  up 

.„  .         when  exposed  to  the  radiations.     This  is 

Scintillations  .  . 

on  Phosphor-   due  to  the  bombardment  ot  the  zinc  sulphide 

escent  Bodies    ^  &  type  Q£  my  ^^  ^  ajpha  ray      Under 

a  magnifying  glass  this  light  is  seen  to  be  made  up  of 
a  number  of  scintillating  points  of  light  and  is  not  con- 
tinuous, each  scintillation  being  of  very  short  duration. 
By  proper  subdivision  of  the  field  under  the  lens,  the 
number  of  scintillations  can  be  counted  with  close 
accuracy. 

A  simple  form  of  apparatus  called  the  spinthariscope 
has  been  devised  to  show  these  scintillations.  A  zinc 
sulphide  screen  is  fixed  in  one  end  of  a  small  tube  and 
a  plate  carrying  a  trace  of  radium  is  placed  very  close 
to  it.  The  scintillations  can  be  observed  through  an 
adjustable  lens  at  the  other  end  of  the  tube.  Outer 
light  should  be  cut  off,  as  in  a  dark  room.  The 
screen  then  appears  to  be  covered  with  brilliant  flashes 
of  light.  Other  phosphorescent  substances,  such  as 
barium  platino-cyanide,  may  be  substituted  for  the 
zinc  sulphide,  but  they  do  not  answer  so  well. 

By  penetrating  power  is  meant  the  power  exhibited 
by  the  rays  of  passing  through  solids  of  different  thick- 
Penetrating  nesses  and  gases  of  various  depths.  This 
Power  power  varies  with  different  radiations  and 

with  the  nature  of  the  solid  or  gas.  For  instance,  a 
sheet  of  metallic  foil  may  be  used  and  the  effect  of 
aluminum  will  differ  from  that  of  gold  and  the  different 


14  RADIO-ACTIVITY 

rays  vary  in  penetrating  power.  In  the  case  of  gases  air 
will  differ  from  hydrogen,  and  it  is  noticed  that  certain 
rays  disappear  after  penetrating  a  short  distance,  while 
others  can  penetrate  further  before  being  lost. 

If  the  radiations  are  subjected  to  the  action  of  a 
strong  magnetic  field,  it  is  found  that  part  of  them 
Magnetic  are  much  deflected  in  the  magnetic  field 
Deflection  an(j  descriDe  circular  orbits,  part  are  only 
slightly  deflected  and  in  the  opposite  direction  from 
the  first,  and  the  remaining  rays  are  entirely  unaffected. 


FIG.  5. — SHOWING  MAGNETIC  DEFLEC- 
TION OF  a,  /3,  AND  7  RAYS. 

By  the  use  of  these  methods  of  investigation  it  is 
learned  that  the  radiations  consist  of  three  types  of 
Three  Types  rays.  These  have  been  named  the  alpha, 
of  Rays  beta,  and  gamma  rays,  respectively.  Some 
radio-active  bodies  emit  all  three  types,  some  two, 
and  some  only  one.  The  distinguishing  characteristic 
of  these  types  of  rays  may  be  summed  up  as  follows : 


PROPERTIES   OF  THE   RADIATIONS         15 

The  alpha  rays  have  a  positive  electrical  charge  and 

a  comparatively  low    penetrating  power.      They  are 

slightly  deflected    in    strong  magnetic  and 

electric  fields.     They  have  a  great  ionizing 

power  and  a  velocity  about  one-fifteenth  that  of  light. 

.    The  beta  rays  are  negatively  charged  and  have  a 

greater  penetrating  power  than  the  alpha  rays.     They 

show  a  strong  deflection  in  magnetic  and 

Beta  Rays  .   .    -  is  i  j  •       •    • 

electric  fields,  have  less  ionizing  power  than 
the  alpha  rays,  and  a  velocity  of  the  same  order  as 
light. 

The  gamma  rays  are  very  penetrating  and  are  not 

deflected  in  the  magnetic  or  electric  fields. 

Gamma  Rays  .       .    . 

Ihey  have  the  least  ionizing  power  and  a 
very  great  velocity. 

The  penetrating,  power  of  each  type  is  complex  and 
varies  with  the  source,  so  the  statements  given  are 
but  generalizations.  The  alpha  rays  are  projected 
particles  which  lose  energy  in  penetrating  matter.  As 
to  the  power  of  ionizing  gases,  if  that  for  the  a  rays 
is  taken  as  10,000,  then  the  /3  rays  would  be  approxi- 
mately 100  and  the  7  rays  1. 

The  rays  are  examined  and  measured  in  several 
ways:  1.  By  their  action  on  the  sensitive  photo- 
Measurement  SraPmc  plates.  The  use  of  this  method  is 
of  Radia-  laborious,  consumes  time,  and  for  com- 
parative measurements  of  intensity  is  un- 
certain as  to  effect. 

2.  By  electrical  methods,  using  electroscopes,  quad- 


16  RADIO-ACTIVITY 

rant  electrometers,  etc.  These  are  the  methods  most 
used. 

3.  By   exposure    to    magnetic    and    electric   fields, 
noting  extent  and  direction  of  deflection. 

4.  By  their  relative  absorption  by  solids  and  gases. 

5.  By  the  scintillations  on  a  zinc  sulphide  screen. 
The  alpha  rays  have  been  identified  as  similar  to  the 

so-called  canal  rays.  These  were  first  observed  in  the 
identification  study  of  the  X  rays.  When  an  electrical 
of  the  Rays  discharge  is  passed  through  a  vacuum  tube 
with  a  cathode  having  holes  in  it,  luminous  streams 
pass  through  the  holes  toward  the  side  away  from  the 
anode  and  the  general  direction  of  the  stream.  They 
travel  in  straight  lines  and  render  certain  substances 
phosphorescent.  These  rays  are  slightly  deflected  by 
a  magnetic  field  and  in  an  opposite  direction  from  that 
taken  by  the  cathode  rays  in  their  deflection.  The 
rays  seem  to  be  positive  ions  with  masses  never  less 
than  that  of  the  hydrogen  atom.  Their  source  is 
uncertain,  but  they  may  be  derived  from  the  electrodes. 

The  beta  rays  are  identical  in  type  with  the  cathode 
rays  and  are  negative  electrons. 

The  gamma  rays  are  analogous  to  the  X  rays  and 
are  of  the  order  of  light.  They  are  in  general  consider- 
ably more  penetrating  than  X  rays.  For  example,  the 
gamma  rays  sent  out  by  30  milligrams  of  radium  can 
be  detected  by  an  electroscope  after  passing  through 
30  centimeters  of  iron,  a  much  greater  thickness  than 
can  be  penetrated  by  the  ordinary  X  rays. 


CHAPTER  III 

CHANGES  IN  RADIO-ACTIVE  BODIES 

Is  this  power  of  emitting  radiations  a  permanent 

property  or  is  it  lost  with  the  passage  of  time?     The 

first  investigations  of  the  activity  of  uranium 

acfa>fty10a         and  thorium  showed  no  loss  of  intensity  at 


Permanent  fae  en(j  of  seVeral  years,  and  radium  also 
seemed  to  show  no  decrease  in  its  enormous 
activity.  Polonium,  however,  was  found  to  lose  most 
of  its  activity  in  a  year,  and  later  it  appeared  that  some 
radio-active  substances  lost  most  of  their  activity  in 
the  course  of  a  few  minutes  or  hours. 

A  phenomenon  called  induced  or  secondary  radio- 
activity was  also  observed.  Thus  a  metal  plate  or 
induced  AC-  wire  exposed  to  the  action  of  thorium  oxide 
tivity  £or  gome  hours  became  itself  active.  This 

induced  activity  was  not  permanent  but  decreased  to 
half  its  value  in  about  eleven  hours  and  practically 
disappeared  within  a  week.  Similar  phenomena  were 
observed  when  radium  was  substituted  for  thorium. 

In  1900  Crookes  precipitated  a  solution  of  an  active 
uranium  salt  with  ammonium  carbonate.  The  pre- 
Discovery  of  cipitate  was  dissolved  so  far  as  possible  in 
Uranium  x  an  excess  of  the  reagent,  leaving  an  insoluble 
residue.  This  residue  was  many  hundred  times  more 


18  RADIO-ACTIVITY 

active,  weight  for  weight,  than  the  original  salt,  and 
the  solution  containing  the  salt  was  practically  inactive. 
At  the  end  of  a  year  the  uranium  salt  had  regained  its 
activity  while  the  residue  had  become  inactive. 

Another  method  of  obtaining  the  same  result  is  to 
dissolve  crystallized  uranium  nitrate  in  ether.  Two 
layers  of  solution  are  formed,  one  ether  and  the  other 
water  coming  from  the  water  of  crystallization.  The 
aqueous  layer  is  active,  while  the  water  layer  is  inactive. 
Similarly,  by  adding  barium  chloride  solution  to  a  solu- 
tion of  a  salt  of  uranium  and  then  precipitating  the 
barium  as  sulphate,  the  activity  is  transferred  to  this 
precipitate.  These  experiments  give  proof  of  the 
formation  and  separation  of  a  radio-active  body  by 
ordinary  chemical  operations. 

So,  too,  in  the  case  of  thorium  salts  a  substance  can 
be  obtained  by  means  of  ammonium  hydroxide  which 
is  several  thousand  times  more  active  than  an  equal 
weight  of  the  original  salt.  After  standing  a  month, 
the  separated  material  has  lost  its  activity  and  the 
thorium  salt  has  regained  it.  Here,  again,  there  is  the 
formation,  separation,  and  loss  of  a  radio-active  body. 

Now,  these  are  ordinary  chemical  processes  for  the 
separation  of  distinct  chemical  individuals.  The  re- 
Conciusions  suits,  therefore,  lead  naturally  to  the  con- 
Drawn  elusions:  (1)  it  would  seem  that  uranium 
and  thorium  are  themselves  inactive  and  the  activity 
is  due  to  some  other  substance  formed  by  these  ele- 
ments; (2)  this  active  substance  is  produced  by  some 


CHANGES  IN  RADIO-ACTIVE  BODIES        19 

transformation  in  those  elements,  for  on  standing  the 
activity  is  regained.  This  latter  conclusion  is  startling, 
for  it  indicates  a  change  in  the  atom  which,  up  to  the 
time  of  this  discovery,  was  deemed  unchangeable  under 
the  influence  of  such  physical  and  chemical  changes  as 
were  known  to  us. 

The  search  for  new  radio-active  bodies  and  the  study 

of  their  characteristics  has  been  systematically  and 

successfully    carried    on.     The   bodies   ob- 

Search  for  .    J 

New  Radio-  tamed  in  the  above  experiments  were 
active  Bodies  named  uranium  x  and  thorium  X,  respec- 
tively. Further,  it  became  clear  from  the  investiga- 
tion of  uranium  minerals  that  radium,  polonium, 
actinium,  and  ionium  originated  from  uranium.  From 
thorium  minerals  a  body  was  separated  called  meso- 
thorium,  which  was  analogous  to  radium.  Both  tho- 
rium and  radium  were  found  to  give  off  a  radio-active 
gas.  The  first  lost  half  of  its  activity  in  less  than  one 
minute.  The  second  was  more  stable  and  lost  half  of 
its  activity  in  about  four  days.  The  name  radium 
emanation  was  given  to  the  latter  and  it  was  found 
chemically  and  physically  to  belong  to  the  class  of  mon- 
atomic  or  noble  gases,  such  as  helium,  argon,  neon,  etc., 
which  had  been  discovered  by  Ramsay.  In  some  cases 
the  chemical  action  was  determined  and  these  new 
bodies  were  found  analogous  to  well-known  elements,  as 
radium  to  barium,  polonium  to  bismuth.  The  physical 
properties  were  investigated  and,  where  possible,  spectra 
were  mapped  and  atomic  weights  determined. 


20  RADIO-ACTIVITY 

It  is  clear,  therefore,  that  these  bodies  are  elemental 
in  character  and  as  such  are  made  up  of  distinct,  similar 
atoms,  just  as  the  commonly  recognized  elements  are 
believed  to  be.  In  this  way  more  than  thirty  new 
elements  have  been  added  to  the  list.  These  new  ele- 
ments are  called  radio-active  elements,  but  it  is  an 
open  question  whether  all  atoms  do  not  possess  this 
property  in  greater  or  less  degree.  Certainly,  it  is 
possessed  in  varying  degree  by  four  of  the  old  elements 
•  widely  separated  in  the  Periodic  System,  namely, 
uranium,  thorium,  rubidium,  and  potassium.  The 
last  two,  while  feebly  active  themselves,  do  not  form 
any  secondary  radio-active  substance  so  far  as  is  known. 
Only  two  of  the  elements,  then,  can  definitely  be  said 
to  go  through  these  transformations.  It  is  just  possible 
that  radio-activity  may  be  found  to  be  a  common 
property  of  all  atoms  and  of  all  matter. 

It  is  important  to  know  how  these  new  bodies  were 
discovered  and  distinguished  from  one  another.  Two 
Methods  of  properties  are  relied  upon.  One  is  the 
investigation  nature  of  the  rays  emitted  and  the  other  is 
the  duration  of  the  activity.  Of  course,  knowledge  of 
the  physical  and  chemical  properties  is  also  of  great 
importance  whenever  obtainable. 

The  nature  of  the  radiation  is  a  distinguishing  charac- 
w  ^  ,  teristic,  though  similarity  here  does  not 

nature  01  »  •  • 

the  Radia-       prove  identity  of  substances.     Some  emit 

a  rays  only,  some  emit  /3  rays,  some  emit 

two  of  the  possible  rays,  as  for  instance,  /3  and  7,  and 


CHANGES   IN   RADIO   ACTIVE-BODIES       21 

some  emit  all  three.  The  rays  may  also  differ  in 
the  velocity  with  which  they  are  emitted  by  different 
radio-active  substances.  Thus,  in  the  case  of  one 
substance  the  a  rays  may  have  a  slightly  greater  or 
less  penetrating  power  than  those  emitted  by  some 
other  substance,  and  this  may  be  true  also  of  the 
other  rays. 

The  duration  of  the  activity  is  called  the  life  period. 

This  is  absolutely  fixed  for  each  body  and  furnishes 

the  most  important  mode  of  differentiating 

Life  Periods  e 

among  them.  It  measures  the  relative 
stability  and  is  the  time  which  must  elapse  before  their 
activity  is  lost  and  they,  changing  into  something 
else,  entirely  disappear.  The  measure  usually  adopted 
is  the  half -value  period.  Two  hypotheses  are  made 
use  of: 

1.  That  there  is  a  constant  production  of  fresh  radio- 
active matter  by  the  radio-active  body. 

2.  That  the  activity  of  the  matter  so  formed  de- 
creases according  to  an  exponential  law  with  the  time 
from  the  moment  of  its  formation. 

These  hypotheses  agree  with  the  experimental  results. 
The  decrease  and  rise  of  activity,  for  example,  of 
uranium  and  uranium  X,  and  also  of  thorium  and  tho- 
rium X,  have  been  measured,  plotted,  and  the  equations 
worked  out. 

Manifestly,  a  state  of  equilibrium  will  be  reached 
when  the  rate  of  loss  of  activity  of  the  matter  already 
produced  is  balanced  by  the  activity  of  the  new  matter 


22  RADIO-ACTIVITY 

produced.  This  equilibrium  and  the  knowledge  of  the 
rate  of  decrease  in  general  will  have  little  value  if  this 
rate,  like  chemical  changes,  is  subject  to  the  influence 
of  chemical  and  physical  conditions.  The  rate  of 
decrease  has  been  found  to  be  unaltered  by  any  known 
chemical  or  physical  agency.  For  instance,  neither 
the  highest  temperatures  applicable  nor  the  cold  of 
liquid  air  have  any  appreciable  effect. 

In  order  to  measure  the  disintegration  of  a  radio- 
active body  in  units  of  time  so  that  the  rate  may  be 
Equilibrium  comparable  with  that  of  other  radio-active 
Series  bodies,  the  relation  between  the  amounts 

under  consideration  must  be  a  definite  one.  For 
this  purpose  equal  weights  of  the  bodies  are  not 
taken,  but  use  is  made  of  the  amounts  which  are 
in  equilibrium  with  a  fixed  amount  of  the  parent 
substance. 

One  gram  of  radium  has  been  settled  upon  as  the 
standard  for  that  series  and  a  unit  known  as  the  "curie" 
has  been  adopted  to  express  the  equilibrium  quantity 
of  radium  emanation.  Thus,  a  curie  of  radium  emana- 
tion (or  niton)  is  the  weight  (or,  as  this  is  a  gas,  the 
volume  at  standard  pressure  and  temperature)  of  the 
emanation  in  equilibrium  with  one  gram  of  radium. 
This,  by  calculation  and  experiment,  is  found  to  be  0.63 
cubic  millimeter.  When  this  amount  has  been  pro- 
duced by  one  gram  of  radium,  the  formation  and  decay 
will  exactly  balance  one  another.  This  is,  therefore, 
one  curie  of  emanation. 


CHANGES   IN   RADIO-ACTIVE   BODIES       23 

The  measurement  of  the  rate  of  decay  is  difficult  but 
can  be  carried  out  with  great  accuracy,  even  down  to 
seconds,  in  the  case  of  certain  short-lived  bodies. 
Errors  crept  in  at  first  from  the  failure  to  completely 
separate  the  substances  produced  in  the  series,  and 
sometimes  because  of  the  simultaneous  production  of 
two  substances. 

As  stated,  the  decay  follows  an  exponential  law. 
The  time  required  for  the  decay  of  activity  to  half- 
value  does  not  mean,  therefore,  that  there  will  be  total 
decay  in  twice  that  time.  Thus  the  half -value  period 
for  uranium  X  is  about  22  days.  The  period  for  com- 
plete decay  is  about  160  days.  This  half -value  period 
corresponds  to  the  half-value  recovery  period  of  ura- 
nium, which  is  also  22  days. 

These  were  the  earlier  figures  obtained  for  uranium 
X  and  they  illustrate  some  of  the  difficulties  surrounding 
such  determinations.  It  was  found  later  that  the  body 
examined  as  uranium  X  was  really  a  constant  mixture 
and  of  course  the  decay  and  recovery  periods  were  also 
composite.  It  required  later  and  very  skilful  work  to 
separate  them  into  the  bodies  indicated  in  the  disinte- 
gration series. 

The  half -value  period  for  thorium  X  is  much  shorter, 
namely,  a  little  over  four  days,  and  this  is  also  the 
recovery  period  for  thorium  X.  The  plotted  decay 
and  recovery  curves  will  intersect  at  this  point. 

The  consecutive  disintegration  series,  with  the  half- 
value  periods,  for  the  uranium  and  thorium  series  as 


24  RADIO-ACTIVITY 

given  by  Soddy  are  seen  in  the  following  tables.  They 
are  probably  subject  to  some  changes  on  further  and 
more  accurate  determination.  The  nature  of  the  rays 
emitted  is  also  given. 


^•v0* 

Thorium             /*~v            ,  x 

Uranium                 (*J  »-n 

(4xl010               pw-'t)->O  ^l7' 

(8  x  109  years)        \-S*O<* 

years  ?)               ^jp 

Uranium  X            (L^J\  ->  •  /3«y 
(35.5  days)             \*_y 
I      »fft) 

^•P\ 

Mesothoriumi   /^^N 
(7.9  years)          \*J 

1 

Ionium                    J!L 
(5x10*  to              ((QUo^ 
10*  years)               \-S 

Actinium  (?)      \) 

Radio-                 x^k 
Actinium            (     J-^O01 
(28.1  days)           J^.^j 

Mesothorium2    /^  .  .  «v  u 
(8.9  hours)          V^y       ^  " 

4 

Radiothorium    /^.^Q, 
(2.91  years  ?)      \_J    u 

Radium                 fC^Uo*. 

Actinium  X        ^^-*O  <x 

Thorium  X        ^"^-^n  ^ 

(2,500  years)         V_y 

(15  days)             x_x 

(5.35  days)         vy 

J 

4 

1 

Emanation            |£M\-»O  ot 
(5.57  days)             Vl/ 

Emanation         ^"\    _ 
(5.6  seconds)      V-/^ 

Emanation         JJ_>Q  oc 
(70  seconds)       v  —  f 

Radium  A             /(Quo  »c 

Actinium  A        ^~\-»n  «x 

Thorium  A         i*\«*O  <?<! 

(4.3  minutes)         \1/ 

(0.0029  second)  W^^ 

(0.203  second)    V^    w 

4 

1 

4, 

Radium  B             (T^)W  •  ft}) 

Actinium  B         [V»»tfJ 

Thorium  B         (j  ->o  ^p) 

(38.5  minutes)        v_x 

(52.  1  minutes)    V^ 
I 

(15.3  hours)         V  — 

Radium  Ci          f/7^-»O  ^ 
(28.1  minutes)    I  \_X^ 

Actinium  Ci    f  (~\+Q  oc 
(3.10mins.)      1  V-/ 

Thorium  Ci      f  f^Uo  «• 
(79  minutes)     1  V_y 

Radium  C2         1  /O\_v   .«, 
(1.9  minutes)      tv*/:  p  / 

Actinium  Cz    1  /^^N^Q  ^ 

Thorium  Cz     1  ^\^Q  ^ 

jT 

1 

1 

Radium  D             ^j»^-*»/i9) 
(24  years  ?)           VHx 

Actinium  D        (j^^^ygexy 
(7.4  minutes)      x_x 

Thorium  D          f~>)-*»/3*y 

(4.5  minutes)      ^x             * 

Radium  E             /C7^i_k*A.w 
(7.25  days)            (^  ->•/*>/ 

Actinium  E         /^~N 
(unknown)       V^y 

I 

Thorium  E         (^\ 
(unknown)          v_x 

Radium  F              Jk 

(Polonium             0»'<>  i))-*  O  <>< 

202  days)               V' 

Radium  G            /f~\ 

(probably             V"'*) 

lead)                       ^^ 

FIG.  6.  —  DISINTEGRATION  SERIES  FOR  URANIUM,  ACTINIUM, 
AND  THORIUM,  AS  GIVEN  BY  SODDY. 


CHAPTER  IV 
NATURE  OF  THE  ALPHA  PARTICLE 

A  THE  remarkable  disintegrations  related  in  the  last 

chapter,  in  which  the  heaviest  known  elementary  atom 

—  that  of  uranium   (at.  wt.   238)  —  is  by 

Dismtegra-  .  v  .  '  J 

tion  of  the  successive  stages  changed  into  others  ot 
lower  atomic  weight,  afford  a  clue  to  the 
nature  of  the  atom  and  to  that  goal  of  the  chemist,  the 
final  constitution  of  matter.  The  composite  nature 
of  the  atom  and  some  sort  of  interrelation  of  the 
elements  had  previously  been  made  apparent  from 
a  study  of  the  Periodic  System  and  data  gathered 
still  earlier,  but  all  attempts  at  working  out  a  so- 
called  genesis  of  the  elements  had  proved  vague  and 
unsatisfactory. 

To  get  an  understanding  of  the  disintegration  occur- 
ring in  radio-active  substances,  the  nature  of  the  rays 
identification  produced  must  be  known.  These'  rays  are 
of  the  Rays  tne  cause  of  the  activity  and  their  emission 
accompanies  the  changes  or  disintegration.  They  have 
for  the  sake  of  convenience  been  called  the  alpha,  beta, 
and  gamma  rays.  The  gamma  rays  have  been  identi- 
fied with  the  X  rays  discovered  by  Rontgen  and  are  a 
form  of  energy  analogous  to  light.  The  beta  rays  are 
particles  of  negative  electricity  or  electrons.  With 


26  RADIO-ACTIVITY 

these,  then,  we  have  some  degree  of  familiarity.  But 
what  are  the  alpha  rays?  An  answer  to  this  question 
should  make  clearer  the  character  of  the  changes  taking 
place,  and  should  give  some  insight  into  the  composition 
and  mechanism  of  the  atom. 

It  has  already  been  stated  that  these  alpha  rays  are 
similar  or  analogous  to  the  canal  rays,  but  this  advances 
The  Alpha  the  matter  very  little,  as  the  nature  of  these 
Rays  canal  rays  has  not  been  fully  determined. 

The  full  identity  with  them,  if  proved,  should  have  an 
important  theoretical  bearing. 

In  the  first  place,  these  alpha  rays  have  been  found 

to  be  made  up  of  solid  particles,  that  is,  of  what  we 

are   accustomed   to   call   matter.     Since  it 

Consist  <>r      has  become  more  and  more  difficult  to  draw 

Solid  Parti-      a    cjear    distinction    between    matter    and 

cles 

energy,  it  would  perhaps  be  better  to  say 
that  these  particles  appear  to  have  some  of  the  prop- 
erties hitherto  attributed  solely  to  matter.  The  best 
evidence  that  these  particles  are  of  atomic  mass  is 
furnished  by  their  deflection  in  electric  and  magnetic 
fields. 

It  is  not  of  first  importance  to  discuss  this  or  other 
proofs  of  the  material  nature  of  these  particles.  That 
Electrical  they  carry  a  charge  of  positive  electricity  is, 
charge  however,  a  fact  of  very  great  import.  The 

value  of  this  charge  has  been  carefully  determined  by 
a  number  of  investigators  working  with  different 
sources  of  the  alpha  particles  and  has  been  found  to 


NATURE  OF  THE  ALPHA  PARTICLE         27 

be 9.3  X  10~10  electrostatic  units  (.000,000,000,93  e.s.). 
From  the  consideration  of  the  charge  upon  an  electron 
previously  obtained  by  J.  J.  Thomson  and  others,  it 
was  concluded  that  the  alpha  particle  carried  two 
unit  positive  charges;  the  fundamental  unit  charge, 
therefore,  is  half  this  value,  or  4.65  X  10~l°  e.s. 

To  determine  the  nature  of  the  alpha  par- 
Formed  from  ticle  a  crucial  experiment  was  carried  out  by 
tides*  Par"  Rutherford  and  Royds,  which  was  described 

as  follows : 

A  large  quantity  of  radium  emanation  was  com- 
pressed into  a  fine  glass  tube  A, 
about  1.5  cm.  long.  This  tube, 
which  was  sealed  to  a  larger  capil- 
lary tube  B,  was  sufficiently  thin 
to  allow  the  alpha  particles  from 
the  emanation  and  its  products  to 
pass  through,  but  sufficiently  thick 
to  withstand  atmospheric  pressure. 
The  thickness  of  the  glass  wall 
was  in  most  cases  less  than  .01  mm. 
On  introducing  the  emanation  into 
the  tube,  the  escape  of  the  alpha 
particles  from  the  emanation  was 
clearly  seen  by  the  scintillations  FlG-  7.  — APPARATUS 

USED  IN  EXPERIMENT 

produced   at    some   distance   on  a      BY  RUTHERFORD  AND 
zinc    sulphide    screen.     After   this      ROYDS. 
test  the  glass  tube  A  was  surrounded  by  a  glass  tube  T 
and  a  small  spectrum  tube  V  attached  to  it.     The  tube 


28  RADIO-ACTIVITY 

T  was  exhausted  to  a  charcoal  vacuum.  By  means  of 
the  mercury  column  H,  the  gases  in  the  tube  T  could 
at  any  time  be  compressed  into  the  spectrum  tube  V 
and  the  nature  of  the  gases  which  had  been  produced 
determined  spectroscopically.  It  was  found  that  two 
days  after  the  introduction  of  the  emanation  into  A 
the  spectrum  showed  the  yellow  line  of  helium,  and 
after  six  days  the  whole  helium  spectrum  was  observed. 
In  order  to  be  certain  that  the  helium,  coming  possibly 
from  some  other  source,  had  not  diffused  through  the 
thin  walls  of  the  tube  A,  the  emanation  was  pumped 
out  and  helium  substituted.  No  trace  of  helium  could 
be  observed  in  the  vacuum  tube  after  several  days, 
showing  that  the  helium  observed  in  the  first  experi- 
ment must  have  originated  from  the  alpha  particles 
which  had  been  propelled  through  the  thin  glass  tube 
into  the  outer  tube.* 

Most  of  the  alpha  particles  are  propelled  with  such 
force  that  they  penetrate  some  distance  into  the  walls 
of  the  outer  tube  and  some  of  these  gradually  diffuse 
out  into  the  exhausted  space.  The  presence  of  helium 
in  the  spectrum  tube  can  be  detected  after  a  shorter 
interval  if  a  thin  cylinder  of  lead  is  placed  over  the 
emanation  tube,  since  the  particles  fired  into  the  lead 
diffuse  out  more  rapidly  than  from  glass. 

A  still  more  definite  proof  of  the  identity  of  the  alpha 
particle  with  the  helium  atom  was  obtained  by  remov- 
ing the  outer  glass  tube  T  and  placing  a  cylinder  of  lead 
over  the  emanation  tube  in  the  open  air.  Helium  was 


NATURE  OF  THE  ALPHA  PARTICLE         29 

always  detected  in  the  lead  after  it  had  remained  sev- 
eral hours  over  the  thin  tube  containing  a  large  quantity 
of  the  emanation.  In  order  to  test  for  the  presence  of 
helium  in  the  lead,  the  gases  present  were  released  by 
melting  the  lead  in  a  closed  vessel.  There  can  thus  be 
no  doubt  that  the  alpha  particle  becomes  a  helium 
atom  when  its  positive  charge  is  neutralized. 

Thus  the  chemist  was  afforded  the  experience  of  the 
building  up  of  at  least  one  element  under  his  observa- 
tion, and  both  the  analysis  and  synthesis  of  matter 
have  been  revealed  through  the  discoveries  of  radio- 
activity. 

It  is  of  interest  at  this  point  to  learn  something  of 
the  history  of  helium  and  its  occurrence.  In  1868 
Discovery  of  there  was  discovered  by  Janssen  and  Lock- 
Helium  ver  a  bright  yellow  line  in  the  spectrum  of 
the  sun's  chromosphere.  Because  of  its  origin  the 
name  helium  was  given  to  the  supposed  new  element 
causing  it.  Later  it  was  found  in  the  spectra  of  many  of 
the  stars,  and  because  of  its  predominance  in  some  of 
these  they  were  called  helium  stars.  Its  existence  on 
our  planet  was  not  detected  for  nearly  thirty  years. 

In  1895,  in  connection  with  the  discovery  of  argon 
in  the  atmosphere,  a  search  was  made  to  see  if  the  latter 
element  could  be  obtained  from  mineral  sources.  In 
analyzing  certain  uranium  minerals  Hillebrand  had 
found  considerable  quantities  of  a  gas  which  he  took  to 
be  a  peculiar  form  of  nitrogen.  Ramsay  made  a  further 
examination  of  the  gas  coming  from  these  minerals  and 


30  RADIO-ACTIVITY 

the  spectroscope  revealed  the  yellow  line  of  helium, 
thus  at  last  proving  the  presence  of  this  element  on  the 
earth.  It  is  known  now  to  be  present  in  thorium  min- 
erals, in  the  waters  of  radio-active  wells,  and  in  minute 
amounts  in  the  atmosphere.  Its  occurrence  in  every 
case,  in  the  light  of  the  experiment  described  above, 
would  seem  to  be  due  to  the  presence  of  radio-active 
changes. 

Helium,  on  account  of  its  chemical  inactivity  and 
physical  properties,  is  classed  along  with  argon,  neon, 
character  krypton,  and  xenon  in  the  zero  group  of  the 
istics  of  Periodic  System,  and  forms  with  them  the 
monatomic,  inert  gases.  In  this  class  are 
now  placed  also  the  three  radio-active  gases,  emanat- 
ing respectively  from  radium,  thorium,  and  actinium. 
These  are  generally  known  as  radium  emanation,  tho- 
rium emanation,  and  actinium  emanation.  The  first 
mentioned  was  once  called  niton.  Emanium  was  the 
name  originally  proposed  by  Giesel  for  the  body  now 
known  as  actinium. 

The  calculated  rate  of  production  of  helium  in  the 
series  in  equilibrium  with  one  gram  of  radium  is  158 
cubic  millimeters  per  year.  This  corresponds  quite 
well  with  the  experimental  results. 

Some  of  the  more  important  atomic  and  radio-active 
constants  are  given  in  the  following  table.  They  are 
Table  of  recorded  here  to  show  how  helpful  the  study 
Constants  of  radio-activity  has  been  in  working  out 
the  composition  of  matter,  and  to  give  some  idea  of 


NATURE  OF  THE  ALPHA  PARTICLE         31 

the  magnitude   of  the   numbers  and  the  minuteness 
of  the  quantities  dealt  with. 

Electric  charge  carried  by  each  H  atom  in 

electrolysis 4.65  x  1Q-10  e.s.1 

Electric  charge  carried  by  each  a  particle     9.3    x  lO"10  e.s. 

Number  of  atoms  in  1  gram  of  H 6.2    x  1023 

Mass  of  1  atom  of  H 1.6    x  10~24  gram 

Number  of  molecules  per  cc.  of  any  gas  at 

standard  pressure  and  temperature 2.72  x  1019 

Number  of  a  particles  expelled  per  second 

per  gram  of  radium  itself 3.6    x  1010 

Number  of  a  particles  expelled  per  second 

per  gram  of  radium  in  equilibrium  with 

its  products 14.3    x  1010 

1  The  expression  10-10  means  multiplying  by  .000,000,000,1;  1010  means 
multiplying  by  10,000,000,000.^ 


CHAPTER  V 
*     THE  STRUCTURE  OF  THE  ATOM 

A  STUDY  of  the  properties  of  radium  will  aid  in  throw- 
ing light  upon  the  question  as  to  the  building  up  of  the 
Properties  of  atom.  First  to  be  considered  are  the  usual 
Radium  properties  which  distinguish  an  elementary 

body.  Metallic  radium  has  been  prepared  by  a  method 
similar  to  that  used  in  the  preparation  of  barium.  It 
is  a  pure  white  metal,  melting  at  700°,  and  far  more 
volatile  than  barium.  It  rapidly  alters  on  exposure  to 
the  air,  probably  forming  a  nitride.  It  energetically 
decomposes  water  and  the  product  dissolves  in  the 
water.  Its  atomic  weight  is  226. 

Radium  forms  a  series  of  salts  analogous  in  appear- 
ance and  chemical  action  to  those  of  barium.  In  the 
course  of  time  they  become  colored,  especially  if  mixed 
barium  salts.  The  radiations  from  radium  produce 
marked  chemical  effects  in  a  number  of  substances. 
Carbon  dioxide  is  changed  into  carbon,  oxygen,  and 
carbon  monoxide,  and  the  latter  is  changed  into  carbon 
and  oxygen.  Ammonia  is  dissociated  into  nitrogen 
and  hydrogen;  hydrochloric  acid  into  chlorine  and 
hydrogen.  Oxygen  is  condensed  into  ozone.  In  gen-« 
eral,  the  action  upon  gases  appears  to  be  similar  to  that' 


THE  STRUCTURE  OF  THE  ATOM  33 

of  the  silent  electric  discharge.  Water  is  decomposed 
into  hydrogen  and  oxygen.  If  moist  radium  chloride 
or  a  salt  of  radium  containing  water  of  crystallization 
is  sealed  in  a  glass  tube,  the  gradual  accumulation  of 
hydrogen  and  oxygen  will  burst  the  tube. 

The  radiations  rapidly  decompose  organic  matter 
•with  the  evolution  of  gases.  Thus  grease  from  stop- 
cocks of  apparatus  used  with  radium  or  paraffin  will 
give  off  carbon  dioxide.  Under  an  intense  alpha  radia- 
tion paraffin  or  vaseline  become  hard  and  infusible. 
White  phosphorus  is  changed  into  red. 

The  action  upon  living  tissue  is  most  noteworthy,  as 
its  possible  use  as  a  remedial  agent  is  dependent  upon 
this.  A  small  amount  of  a  radium  salt  enclosed  in  a 
glass  tube  will  cause  a  serious  burn  on  flesh  exposed  to 
it.  It  therefore  has  to  be  handled  with  care  and  undue 
exposure  to  the  radiations  must  be  avoided.  Cancer 
sacs  shrivel  up  and  practically  disappear  under  its 
action.  WThether  the  destruction  of  whatever  causes 
the  cancer  is  complete  is  at  least  open  to  serious  doubt. 

The  coagulating  effect  upon  globulin  is  interesting. 
When  two  solutions  of  globulin  from  ox  serum  are 
taken  and  acetic  acid  added  to  one  while  ammonia  is 
added  to  the  other,  the  opalescence  in  drops  of  the 
former  is  rapidly  diminished  on  exposure  to  radium, 
showing  a  more  complete  solution,  whereas  the  latter 
solution  rapidly  turns  to  a  jelly  and  becomes  opaque, 
indicating  a  greatly  decreased  solubility. 

The  greater  part  of  the  tremendous  energy  evolved 


34  RADIO-ACTIVITY 

by  radium  is  due  to  the  emission  of  the  alpha  particles, 
Energy  anc^  m  comParison  the  beta  and  gamma  rays 

Evolved  by  together  supply  only  a  small  fraction.  This 
energy  may  be  measured  as  heat.  It  was 
first  observed  that  a  radium  compound  maintained  a 
temperature  several  degrees  higher  than  that  of  the 
air  around  it.  The  rate  of  heat  production  was  later 
measured  by  means  of  an  ice  calorimeter  and  also  by 
noting  the  strength  of  the  current  required  to  raise  a 
comparison  tube  of  barium  salt  to  the  same  tempera- 
ture. Both  methods  showed  that  the  heat  produced  was 
at  the  rate  of  about  135  gram  calories  per  hour.  As 
the  emission  is  continuous,  one  gram  of  radium  would 
therefore  emit  about  1,180,000  gram  calories  in  the 
course  of  a  year.  At  the  end  of  2000  years  it  would 
still  emit  590,000  gram  calories  per  year.  Such  a  pro- 
duction of  energy  so  far  surpasses  all  experience  that 
it  becomes  almost  inconceivable.  It  is  futile  to  speak 
of  it  in  terms  of  the  heat  evolved  by  the  combustion 
of  hydrogen,  which  is  the  greatest  that  can  be  produced 
by  chemical  means. 

This  effect  is  unaltered  at  low  temperatures,  as  has 
been  tested  by  immersing  a  tube  containing  radium  in 
liquid  air.  It  should  be  stated  that  these  measure- 
ments were  made  after  the  radium  had  reached  an 
equilibrium  with  its  products;  that  is,  after  waiting  at 
least  a  month  after  its  preparation.  The  evolution  of 
heat  from  radium  and  the  radio-active  substances  is, 
in  a  sense,  a  secondary  effect,  as  it  measures  the  radiant 


THE  STRUCTURE  OF  THE  ATOM  35 

energy  transformed  into  heat  energy  by  the  active  mat- 
ter itself  and  whatever  surrounds  it.  Let  us  repeat, 
therefore,  that  the  total  amount  of  energy  pent  up  in 
a  single  atom  of  radium  almost  passes  our  powers  of 
conception. 

The  facts  gathered  so  far  justify  and  necessitate  a 

theory  which   shall   satisfactorily  explain   them,    and 

since  these  phenomena  are  not  caused  by 

Necessity  for  r 

a  Disintegra-  nor  subject  to  the  influence  of  external 
iory  agencies,  they  must  refer  to  changes  taking 
place  within  the  atom  —  in  other  words,  a  theory  of 
disintegration.  In  the  main,  these  facts  may  be 
summed  up  as  the  emission  of  certain  radiations  from 
known  elemental  matter:  the  material  alpha  particles 
with  positive  charge,  the  beta  particles  or  negative 
electrons,  and  the  gamma  rays  analogous  to  X  rays. 
The  emission  of  these  rays  results  in  the  production  of 
great  heat.  Then  there  is  the  law  of  transformations  ~ 
by  which  whole  series  of  new  elements  are  generated 
from  the  original  element  and  maintain  a  constant  equi-  ~ 
librium  of  growth  and  decay  in  the  series.  Lastly,  we 
have  the  production  of  helium  from  the  alpha  particles.  *' 

In  explanation  of  these  phenomena,  Rutherford 
offered  the  hypothesis  that  the  atoms  of  certain  ele- 
Disintegra-  ments  were  unstable  and  subject  to  dis- 
tion  Theory  integration.  The  only  elements  definitely 
known  to  come  under  this  description  are  the  two 
having  atoms  of  the  greatest  known  mass,  thorium 
(232)  and  uranium  (238). 


36  RADIO-ACTIVITY 

The  atoms  of  uranium,  for  instance,  are  supposed  to 
be  not  permanent  but  unstable  systems.  According 
to  the  hypothesis,  about  1  atom  in  every  1018  becomes 
unstable  each  second  and  breaks  up  with  a  violent 
explosion  for  so  small  a  mass  of  matter.  One,  or  pos- 
sibly two  alpha  particles  are  expelled  with  great  veloc- 
ity. This  alpha  particle  corresponds  to  an  atom  of 
helium  with  an  atomic  weight  of  4,  and  its  loss  reduces 
the  original  atomic  weight  to  234  with  the  formation 
of  a  new  element,  having  changed  properties  corre- 
sponding to  the  new  atomic  weight.  This  new  element 
is  uranium  Xi. 

These  new  atoms  are  far  more  unstable  than  those 
of  uranium,  and  the  decomposition  proceeds  at  a  new 
rate  of  1  in  107  per  second.  So  at  a  definite,  measurable 
rate  this  step  wise  disintegration  proceeds.  The  explo- 
sions are  not  in  all  cases  equally  violent  in  going  from 
element  to  element,  nor  are  the  results  the  same. 
Sometimes  alpha  particles  alone  are  expelled,  some- 
times beta,  or  two  of  them  together,  as  alpha  and  beta. 

The  new  product  may  remain  with  the  unchanged 
part  of  the  original  matter.  Thus  there  would  be  an 
accumulation  of  it  until  its  own  decay  balances  its  pro- 
duction, resulting  eventually  in  a  state  of  equilibrium. 

In  order  to  explain  the  electrical  and  optical  prop- 
erties of  matter,  the  hypothesis  was  made  that  the 
Constitution  atom  consisted  of  positively  and  negatively 
of  the  Atom  electrified  particles.  Later  it  was  shown 
that  negative  electrons  exist  in  all  kinds  of  matter. 


THE  STRUCTURE  OF  THE  ATOM  37 

Various  attempts  were  made  to  work  out  a  model  of 
such  an  atom  in  which  these  particles  were  held  in 
equilibrium  by  electrical  forces.  The  atom  of  Lord 
Kelvin  consisted  of  a  uniform  sphere  of  positive  elec- 
trification throughout  which  a  number  of  negative 
electrons  were  distributed,  and  J.  J.  Thomson  has 
determined  the  properties  of  this  type  as  to  the  num- 
ber of  particles,  their  arrangement  and  stability. 

According  to  Rutherford,  the  atom  of  uranium  may 
be  looked  upon  as  consisting  of  a  central  charge  of 
Rutherford's  positive  electricity  surrounded  by  a  number 
Atom  Q£  concentric  rings  of  negative  electrons  in 

rapid  motion.  The  positively  charged  centre  is  made 
up  of  a  complicated  system  in  movement,  consisting 
in  part  of  charged  helium  and  hydrogen  atoms,  and 
practically  the  whole  charge  and  mass  of  the  atom  is 
concentrated  at  the  centre.  The  central  system  of  the 
atom  is  from  some  unknown  cause  unstable,  and  one 
of  the  helium  atoms  escapes  from  the  central  mass  as 
an  alpha  particle. 

There  are,  confessedly,  difficulties  connected  with 
this  conception  of  the  atom  which  need  not,  however, 
be  discussed  here.  Much  remains  to  be  learned  as  to 
the  mechanics  of  the  atom,  and  the  hypothesis  outlined 
above  will  probably  have  to  be  materially  altered  as 
knowledge  grows.  Perhaps  it  may  have  to  be  entirely 
abandoned  in  favor  of  some  more  satisfactory  solution. 
Until  such  time  it  at  least  suffices  as  a  mental  picture 
around  which  the  known  facts  group  themselves.  In 


38  RADIO-ACTIVITY 

this  picture  energy  and  matter  lose  their  old-time 
distinctness  of  definition.  Discrete  subdivisions  of 
energy  are  recognized  which  may  be  called  charged 
particles  without  losing  their  significance.  Some  of 
these  subdivisions  charged  in  a  certain  way  or  with 
neutralized  charge  exhibit  the  properties  of  so-called 
matter. 

This  conception  of  the  atom  would  doubtless  fail 
of  much  support  were  it  not  for  certain  experimental 
Scattering  of  ^acts  which  lend  great  weight  to  it.  Certain 
Alpha  Par-  suppositions  can  be  based  on  this  theory 
mathematically  reasoned  out  and  tested  by 
experiment.  Predictions  thus  based  on  mathematical 
reasoning  and  afterward  confirmed  by  experiment  give 
a  very  convincing  impression  that  truth  lies  at  the 
bottom. 

The  first  of  these  experimental  proofs  comes  under 
the  head  of  what  is  known  as  the  scattering  of  the  alpha 
particles,  a  phenomenon  which,  when  first  observed, 
proved  hard  to  explain.  If  an  alpha  particle  in  its 
escape  from  the  parent  atom  should  come  within  the 
influence  of  the  supposed  outer  electrical  field  of  some 
other  atom,  it  should  be  deflected  from  its  course  and, 
the  intensity  of  the  two  charges  being  known,  the 
angle  of  deflection  could  be  calculated.  For  instance, 
if  it  came  to  what  might  be  called  a  head-on  collision 
with  the  positive  central  nucleus  of  another  atom,  it 
would  recoil  if  it  were  itself  of  lesser  mass,  or  would 
propel  the  other  forward  if  that  were  the  lighter. 


THE  STRUCTURE  OF  THE  ATOM  39 

The  experiment  is  carried  out  by  placing  a  thin  metal 
foil  over  a  radio-active  body,  as  radium  C,  which  expels 
alpha  particles  with  a  high  velocity,  and  counting  the 
number  of  alpha  particles  which  are  scattered  through 
an  angle  greater  than  90°  and  so  recoil  toward  their 
source.  This  has  been  done  by  a  number  of  investi- 
gators and  it  has  been  found  that  the  angle  of  scattering 
and  the  number  of  recoil  particles  'depend  upon  the 
atomic  weight  of  the  metal  used  as  foil.  For  example, 
if  gold  is  used,  the  number  of  recoil  atoms  is  one  in  some- 
thing less  than  8,000. 

Taking  the  atomic  weight  of  gold  into  consideration, 
Rutherford  calculated  mathematically  that  this  was 
about  the  number  which  should  be  driven  backward. 
But  he  went  further  and  calculated  also  the  number 
which  should  be  returned  by  aluminum,  which  has  an 
atomic  weight  of  only  about  one-seventh  that  of  gold. 
Two  investigators  determined  experimentally  the  num- 
ber for  aluminum  and  their  results  agreed  with  Ruther- 
ford's calculations. 

The  metals  from  aluminum  to  gold  have  been  ex- 
amined in  this  way.  The  number  of  recoil  particles 
increases  with  the  atomic  weight  of  the  metal.  Com- 
paring experiment  with  theory,  the  central  charge  in 
an  atom  corresponds  to  about  one-half  the  atomic 
weight  multiplied  by  the  charge  on  an  electron,  or,  as  it 
is  expressed,  J  Ae.  v 

There  is  only  one  lighter  atom  than  helium,  namely, 
hydrogen,  which  has  a  mass  only  one-fourth  as  great. 


40  RADIO-ACTIVITY 

When  alpha  particles  are  discharged  into  hydrogen,  a 
few  of  the  latter  atoms  are  found  to  be  propelled  to  a 
distance  four  times  as  great  as  that  reached  by  the 
alpha  particles. 

Parallel  with  the  experiments  mentioned,  there  is 
what  is  called  the  stopping  power  of  substances.  This 
stopping  means  the  depth  or  thickness  of  a  substance 
Power  of  necessary  to  put  a  stop  to  the  course  of  the 
alpha  particles.  This  gives  the  range  of 
the  alpha  particles  in  such  substances  and  is  connected 
in  a  simple  way  with  the  atomic  weight,  that  is,  it  is 
again  fixed  by  the  mass  of  the  opposing  atom.  This 
stopping  power  of  an  atom  for  an  alpha  particle  is 
approximately  proportional  to  the  square  root  of  its 
atomic  weight. 

Considering  gases,  for  instance,  if  the  range  in  hydro- 
gen be  1,  then  the  range  in  oxygen,  the  atomic  weight 
of  which  is  16,  is  only  V^  or  J.  Generally  in  the  case 
of  metals  the  weight  of  matter  per  unit  area  required 
to  stop  the  alpha  particle  is  found  to  vary  accord- 
ing to  the  square  root  of  the  atomic  weight  of  the 
metal  taken. 


CHAPTER  VI 

RADIO-ACTIVITY  AND  CHEMICAL  THEORY 

IT  can  easily  be  seen  that  the  revelations  of  radio- 
activity must  have  a  far-reaching  effect  upon  chemical 
theory,  throwing  light  upon,  and  so  bringing 
upon  chem-  nearer,  the  solution  of  some  of  the  problems 
tory  which  have  been  long  discussed  without 
arriving  at  any  satisfactory  solution.  The  so-called 
electro-chemical  nature  of  the  elements  will  certainly 
be  made  much  clearer.  The  changes  in  valence  should 
become  intelligible  and  valence  itself  should  be  ex- 
plained. A  fuller  understanding  of  the  ionization  of 
electrolytes  also  becomes  possible.  As  these  matters 
are  debatable  and  the  details  are  still  unsettled,  it  is 
scarcely  appropriate  to  give  here  the  hypotheses  in 
detail  or  to  enter  into  any  discussion  of  them.  But 
the  promise  of  solution  in  accord  with  the  facts  is 
encouraging. 

Such  progress  has  been  made,  however,  in  regard  to 
a  better  understanding  of  the  Periodic  System  that  the 
The  Periodic  n^w  facts  and  their  interpretation  may  well 
System  j^  given  NO  reliable  clue  to  the  meaning 

of  this  system  and  the  true  relationship  between  the 


42  RADIO-ACTIVITY 

elements  had  been  found  up  to  the  time  when  new 
light  was  thrown  upon  it  by  the  discoveries  of  radio- 
activity. The  underlying  principle  was  unknown  and 
even  the  statement  of  what  was  sometimes  erroneously 
called  the  Periodic  Law  was  manifestly  incorrect  and 
its  terms  were  ignored. 

The  ordinary  statement  of  the  fundamental  prin- 
ciple of  the  Periodic  System  has  been  that  the 
Basis  of  the  Pr°Perties  of  the  elements  were  periodic 
Periodic  Sys-  functions  of  the  atomic  weights,  and  that 
when  the  elements  were  arranged  in  the 
order  of  their  atomic  weights  they  fell  into  a  natural 
series,  taking  their  places  in  the  proper  related 
groups. 

In  accepting  this,  the  interpretation  of  function  was 
both  unmathematical  and  vague,  and  the  order  of  the 
atomic  weights  was  not  strictly  adhered  to  but  unhesi- 
tatingly abandoned  to  force  the  group  relationship. 
Wherever  consideration  of  the  atomic  weight  would 
have  placed  an  element  out  of  the  grouping  with 
other  elements  to  which  it  was  clearly  related  in 
physical  and  chemical  properties,  the  guidance  of  these 
properties  was  accepted  and  that  of  the  atomic 
weights  disregarded.  Such  shif tings  are  noted  in  the 
cases  of  tellurium  and  iodine;  cobalt  and  nickel; 
argon  and  potassium.  It  was  most  helpful  that,  fol- 
lowing the  order  of  atomic  weights,  the  majority  of 
the  elements  fell  naturally  into  their  places.  Otherwise 
the  generalization  known  as  the  Periodic  System  might 


RADIO-ACTIVITY  AND  CHEMICAL  THEORY    43 

have  remained  for  a  long  time  undiscovered  and 
the  progress  of  chemistry  would  have  been  greatly 
retarded. 

It  is  evident  that  the  order  of  the  elements  is  deter- 

mined by  something  else  than  their  atomic  weights. 

From  the  known  facts  of  radio-activity  it 

Influence  of  .,..„. 

Positive  would  seem  that  this  determining  factor  is 

the  positive  nucleus.  And  this  nucleus  also 
determines  the  mass  or  weight  of  the  atom.  Taking 
the  elements  in  their  order  in  the  Periodic  Series  and 
numbering  the  positions  held  by  them  in  this  series  as 
1,  2,  3,  etc.,  we  get  the  position  number  or  what  is 
called  the  atomic  number.  This  designates  the  order  or 
position  of  the  element  in  the  series.  We  must  learn 
that  this  number  marks  a  position  rather  than  a 
single  element,  a  statement  which  will  be  explained 
later. 

Since  the  atomic  weight  is  unreliable  as  a  means  of 

settling  the  position  of  an  element  in  the  series  and  so 

fixing  its  atomic  number,  how  is  this  number 

Determina-  ,     •          .        ,*      >-.» 

turn  of  the       to  be  determined  :     (Jt  course,  one  answer 


Nmnber  to  ^s  question  is  that  we  may  rely  upon  a 
consideration  of  the  general  properties,  as 
has  been  done  in  the  past.  Fortunately,  other  methods 
have  been  found  by  which  this  may  be  confirmed. 
For  instance,  the  stopping  and  scattering  power  of  the 
element  for  alpha  particles  has  been  suggested  and  suc- 
cessfully used. 

A    most   interesting   method    is    due   to    Moseley's 


44  RADIO-ACTIVITY 

observations  upon  the  X-ray  spectra  of  the  various 
Use  of  x-  elements.  It  has  been  found  that  crystals, 
Ray  Spectra  sucn  as  those  of  quartz,  have  the  power  of 
reflecting  and  defining  the  X  rays.  The  spectra  given 
by  these  rays  can  be  photographed  and  the  wave  lengths 
measured.  These  X  rays  are  emitted  by  various  sub- 
stances under  bombardment  by  the  cathode  rays 
(negative  electrons)  and  have  great  intensity  and  very 
minute  wave  lengths.  Moseley  made  use  of  various 
metals  as  anti-cathodes  for  the  production  of  these 
rays.  These  metals  ranged  from  calcium  to  zinc  in 
the  Periodic  System.  In  each  case  he  observed  that 
two  characteristic  types  of  X  rays  of  definite  intensity 
and  different  wave  lengths  were  emitted.  From  the 
frequency  of  these  waves  there  is  deduced  a  simple 
relation  connected  with  a  fundamental  quantity  which 
increases  in  units  from  one  element  to  the  next.  This 
is  due  to  the  charge  of  the  positive  central  nucleus. 
The  number  found  in  this  way  is  one  less  than  the 
atomic  number.  Thus  the  number  for  calcium  is  19 
instead  of  20  and  that  for  zinc  is  29  instead  of  30.  So, 
by  adding  1  to  the  number  found  the  atomic  number  is 
obtained. 

The  atomic  weight  can  usually  be  followed  in  fixing 
the  atomic  number,  but  where  doubt  exists  the  method 
just  given  can  be  resorted  to.  Thus  doubt  arises  in 
the  case  of  iron  and  nickel  and  cobalt.  This  would  be 
the  order  according  to  the  atomic  weights.  The 
X-ray  method  gives  the  order  as  iron,  cobalt,  and 


RADIO-ACTIVITY  AND  CHEMICAL  THEORY    45 

nickel,  and  this  is  the  accepted  order  in  the  Periodic 
System. 

On  studying  the  properties  of  the  elements  in  a  trans- 
formation series  in  connection  with  the  ray  emission 
which  produced  them,  it  was  seen  that  these 

Changes  \  '    . 

Caused  by       properties  were  determined  in  each  case  by 

Ray  Emission    ^    nature    Qf    ^    rfty    emiUed    from    the 

preceding  transformation  product  or  parent  element. 

Each  alpha  particle  emitted  means  a  loss  of  4  in  the 

atomic  weight.     This  is  the  mass  of  a  helium  atom. 

Thus  from  uranium  with  an  atomic  weight 

Atomic  . 

Weight  of  238  to  racjium  there  is  a  loss  ot  three 

alpha  particles.  Therefore,  12  must  be 
subtracted  from  238,  leaving  226,  which  agrees  closely 
with  the  atomic  weight  of  radium  as  actually  determined 
by  the  ordinary  methods.  Uranium  Xi,  then,  would 
have  an  atomic  weight  of  234  and  that  of  ionium  would 
be  230.  The  other  intermediate  elements,  whose  forma- 
tion is  due  to  the  loss  of  beta  particles  only,  show  no 
decrease  in  atomic  weight. 

From  uranium  to  lead  there  is  a  loss  of  8  alpha  par- 
ticles, or  32  units  in  atomic  weight.  This  would  give 
Lead  the  for  the  final  product  an  atomic  weight  of 
End  Product  206  The  atomic  weight  of  lead  is  207.17. 
It  is  not  at  all  certain  that  the  final  product  of  this 
series  is  ordinary  lead.  The  facts  are  such  that  they 
would  lead  one  to  think  that  it  is  not.  It  is  known 
only  that  the  end  product  would  probably  be  some  ele- 
ment closely  resembling  lead  chemically  and  hence 


46  RADIO-ACTIVITY 

difficult  or  impossible  to  separate  from  it.  Several 
accurate  determinations  of  lead  coming  from  uranium 
minerals,  which  always  carry  this  element  and  in  an 
approximately  definite  ratio  to  the  amount  of  uranium 
present,  show  atomic  weights  of  206.40;  206.36;  and 
206.54.  Even  the  most  rigid  methods  of  purification 
fail  to  change  these  results.  The  lead  in  these  minerals 
might  therefore  be  considered  as  coming  in  the  main 
from  the  disintegration  of  the  uranium  atom  and, 
though  chemically  resembling  lead,  as  being  in  reality 
a  different  element  with  different  atomic  weight. 

Furthermore,  in  the  thorium  series  6  alpha  particles 
are  lost  before  reaching  the  end  product,  which  again 
is  perhaps  the  chemical  analogue  of  lead.  The  atomic 
weight  here  should  be  232  less  24,  or  208.  Determina- 
tions of  the  atomic  weight  of  lead  from  thorite,  a  tho- 
rium mineral  nearly  free  from  uranium,  gave  208.4. 

The  end  product  of  the  actinium  series  is  also  an 
element  resembling  lead,  but  both  the  beginning  and 
ending  of  this  series  are  still  in  obscurity. 

The  loss  of  4  units  in  the  atomic  weight  of  an  element 

on   the  expulsion   of   an  alpha   particle   is 

Position  in       accompanied    by    a    change    of    chemical 

the  Periodic     properties  which  removes  the  new  element 

System  » »     .        i 

two  groups  toward  the  positive  side  in  the 
Periodic  System. 

Thus  ionium  is  so  closely  related  to  thorium  and  so 
resembles  it  chemically  that  it  is  properly  classed  along 
with  thorium  as  a  quadrivalent  element  in  the  fourth 


RADIO-ACTIVITY  AND  CHEMICAL  THEORY    47 

group.  Ionium  expels  an  alpha  particle  and  becomes 
radium,  which  is  a  bivalent  element  resembling  barium 
belonging  to  the  second  group.  Radium  then  expels 
an  alpha  particle  and  becomes  the  gas,  radium  emana- 
tion, which  is  an  analogue  of  argon  and  belongs  to  the 
zero  group.  Other  instances  might  be  cited  which  go 
to  show  that  in  all  cases  the  loss  of  an  alpha  particle 
makes  a  change  of  two  places  toward  the  left  or  positive 
side  of  the  System. 

The  loss  of  a  beta  particle  causes  no  change  in  the 

atomic  weight  but  does  cause  a  shift  for  each  beta  par- 

ticle of  one  group  toward  the  right  or  nega- 

from  LOSS  of    tive  side  of  the  System.     Two  such  losses, 


"  then,  will  counterbalance  the  loss  of  an 
alpha  particle  and  bring  the  new  element 
back  to  the  group  originally  occupied  by  its  progenitor. 
Thus  uranium  in  the  sixth  group  loses  an  alpha  particle 
and  the  product  UXi  falls  in  the  fourth  group.  One 
beta  particle  is  then  lost  and  UX2  belonging  to  the 
fifth  group  is  formed.  With  the  loss  of  one  more  beta 
particle  the  new  element  returns  to  the  sixth  group  from 
which  the  transformation  began. 

The  table  on  page  48,  as  adapted  from  Soddy,  affords 
a  general  view  of  these  changes. 

An  examination  of  the  table  will  show  a  number  of 

different  elements  falling  in  the  same  position  in  a 

isoto  es          group  of  the  Periodic  System  irrespective 

of  their  atomic  weights.     These  are  chem- 

ically inseparable  so  far  as  the  present  limitations  of 


48  RADIO-ACTIVITY 


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RADIO-ACTIVITY  AND   CHEMICAL  THEORY    49 

chemical  analysis  are  concerned.  Even  the  spectra 
of  these  elements  seem  to  be  identical  so  far  as 
known.  This  identity  extends  to  most  of  the  physical 
properties,  but  this  demands  much  further  investi- 
gation. For  this  new  phenomenon  Soddy  has  sug- 
gested the  word  isotope  for  the  element  and  isotopic 
for  the  property,  and  these  names  have  come  into 
general  use. 

Manifestly,  we  have  come  across  a  phenomenon  here 
which  quite  eliminates  the  atomic  weight  as  a  deter- 
mining factor  as  to  position  in  the  Periodic  or  Natural 
System  or  of  the  elemental  properties  in  general.  All 
of  the  properties  of  the  bodies  which  we  call  elements, 
and  consequently  of  their  compounds  and  hence  of 
matter  in  general,  seem  to  depend  upon  the  balance 
maintained  between  the  charges  of  negative  and  positive 
electricity  which,  according  to  Rutherford's  theory,  go 
to  make  up  the  atom. 

It  is  evident  that  any  study  of  chemical  phenomena 
and  chemical  theory  is  quite  incomplete  without  a 
study  of  radio-activity  and  the  transformations  which 
it  produces. 

In  concluding  this  outline  of  the  main  facts  of  radio- 
activity, it  is  of  interest  to  discuss  briefly  the  presence 
Radio-activ-  of  radio-active  material  on  this  planet  and 
ity  in  Nature  in  the  stars>  Facts  enougn  have  been 

gathered  to  show  the  probable  universality  of  this 
phenomenon  of  radio-activity.  Whether  this  means 
solely  the  disintegration  of  the  uranium  and  thorium 


50  RADIO-ACTIVITY 

atoms,  or  whether  other  elements  are  also  transformed 
under  the  intensity  of  the  agencies  at  work  in  the  uni- 
verse, is  of  course  a  question  as  yet  unsolved. 

The  presence  of  uranium  and  thorium  widely  dis- 

tributed throughout  the  crust  of  the  earth  would  lead 

to  the  conclusion  that  their  disintegration 

Products  in      products  would  be  found  there  also.     Vari- 


ous  rocks  of  igneous  origin  have  been  exam- 
ined revealing  from  4.78  X  10~12  to  0.31 
X  10  ~12  grams  of  radium  per  gram  of  the  rock.  Aque- 
ous rocks  have  shown  a  lesser  amount,  ranging  from 
2.92  X  10-12  to  0.86  X  10-12  grams.  As  the  soil  is 
formed  by  the  decomposition  of  these  rocks,  radium  is 
present  in  varying  amounts  in  all  kinds  of  soil. 

As  radium  is  transformed  into  the  gaseous  emanation, 
this  will  escape  wherever  the  soil  is  not  enclosed.  For 
~  n  .  instance,  a  larger  amount  of  radio-activity 
Air  and  Soil  is  found  in  the  soil  of  caves  and  cellars  than 
in  open  soils.  If  an  iron  pipe  is  sunk  into 
a  soil  and  the  air  of  the  soil  sucked  up  into  a  large  elec- 
troscope, the  latter  instrument  will  show  the  effect  of 
the  rays  emitted  and  will  measure  the  degree  of  activity. 
Also  the  interior  of  the  pipe  will  receive  a  deposit  of 
the  radio-active  material  and  will  show  appreciable 
radio-activity  after  being  removed  from  the  soil. 

This  radium  emanation  is  dissolved  in  the  soil  waters, 
wells,  springs,  and  rivers,  rendering  them  more  or  less 
radio-active,  and  sometimes  the  muddy  deposit  at  the 
bottom  of  a  spring  shows  decided  radio-activity. 


RADIO-ACTIVITY  AND  CHEMICAL  THEORY     51 

The  emanation  also  escapes  into  the  air  so  that  many 
observations  made  in  various  places  show  that  the 
radium  emanation  is  everywhere  present  in  the  atmos- 
phere. Neither  summer  nor  winter  seems  to  affect 
this  emanation,  and  it  extends  certainly  to  a  height  of 
two  or  three  miles.  Rain,  falling  through  the  air,  dis- 
solves some  of  the  emanation,  so  that  it  may  be  found 
in  freshly-fallen  rain  water  and  also  in  freshly-fallen 
snow.  Radio-active  deposits  are  found  upon  electri- 
cally charged  wires  exposed  near  the  earth's  surface. 

As  helium  is  the  resulting  product  of  the  alpha  par- 
ticles emitted  by  the  emanation  and  other  radio-active 
bodies,  it  is  found  in  the  soil  air,  soil  waters,  and 
atmosphere. 

Average  measurements  of  the  radio-activity  of  the 
atmosphere  have  led  to  the  calculation  that  about  one 
gram  of  radium  per  square  kilometer  of  the  earth's  sur- 
face is  requisite  to  keep  up  the  supply  of  the  emanation.. 

A  number  of  estimates  have  been  given  as  to  the 
heat  produced  by  the  radio-active  transformations  going 
on  in  the  material  of  this  planet.  Actual  data  are 
scarce  and  mere  assumptions  unsatisfactory,  so  little 
that  is  worth  while  can  be  deduced.  It  is  possible  that 
this  source  of  heat  may  have  an  appreciable  effect  upon 
or  serve  to  balance  the  earth's  rate  of  cooling. 

Meteorites  of  iron  coming  from  other  celestial  bodies 
Cosmicai  have  not  shown  the  presence  of  radium. 
Radio-activity  Aerolites  or  stone  meteorites  have  been 
found  to  contain  as  much  as  similar  terrestrial  rock. 


52  RADIO-ACTIVITY 

Since  the  sun  contains  helium  and  some  stars 
show  its  presence  as  predominating,  this  suggests  the 
presence  of  radio-active  matter  in  these  bodies.  In 
addition,  the  spectral  lines  of  uranium,  radium,  and 
the  radium  emanation  have  been  reported  as  being 
found  in  the  sun's  spectrum  and  also  in  the  new  star, 
Nova  Geminorum  2.  These  observations  await  further 
investigation  and  confirmation.  So  far  as  the  sun's 
chromosphere  is  concerned,  the  possible  amount  of 
radium  present  would  seem  to  be  very  small.  If  this 
is  true,  radio-active  processes  could  have  little  to  do 
with  the  sun's  heat.  The  statement  is  made  by 
Rutherford  that  indirect  evidence  obtained  from  the 
study  of  the  aurora  suggests  that  the  sun  emits  rays 
similar  in  type  to  the  alpha  and  beta  rays.  Such  rays 
would  be  absorbed,  and  the  gamma  rays  likewise,  in 
passing  through  the  earth's  atmosphere  and  so  escape 
ordinary  observation.  All  of  this  is  but  further  evi- 
dence of  the  unity  of  matter  and  of  forces  in  the 
universe. 


INDEX 


Actinium,  discovery  of,  6 
Activity,  induced,  17 
Alpha   particles,    effect   of   loss   on 
Atomic  Weight,  45 

electrical  charge  of,  26 

form  helium,  27 

nature  of,  25 

penetrating  power  of,  39 

position  of  element  changed  by 
its  loss,  46 

recoil,  39 

scattering  of,  38 

solid,  26 
Atom,  constitution  of,  36 

Kelvin's,  37 

models  of,  37 

Rutherford's,  37 
Atomic  number,  determination  of,  43 

Becquerel's  experiments,  2 
Beta  particles,  change  in  position  of 
element  by  loss  of,  47 

Chalcolite,  natural  and  artificial,  4 
Constants,  table  of,  31 
Curie  unit,  22 

Disintegration  of  the  element,  25 
Disintegration  series,  24 
Disintegration  theory,  35 

Electroscope,  12 
Equilibrium  series,  22 


Helium,  characteristics  of,  30 
discovery  of,  29 

Ionium,  discovery  of,  6 
lonization,    application    of    electric 
field  to,  10 

experimental  confirmation,  9 
lonization  of  gases,  7 

theory  of,  8 

Ions,  size  and  nature  of,  10 
Isotopes,  47 

Lead,    atomic   weight    varies   with 

source,  45 
radio-active,  6 
the  end  product,  45 
Life-periods  of   radio-active  bodies, 
21 

Periodic  system,  41 

basis  of,  42 

Polonium,  discovery  of,  4 
Positive  nucleus,  influence  of,  43 
Potassium,  radio-activity  of,  3 

Radiations,   action  on  phosphores- 
cent bodies,  13 

action  on  photographic  plates,  11 
discharge  electrified  bodies,  12 
magnetic  deflection  of,  14 
measurements  of,  15 
penetrating  power  of,  13,  15 


54 


. 

Radio-active  bodies,  elemental  na- 
ture of,  20 

examination  of,  20 

life  periods  of,  21 
Radio-activity,  an  atomic  property,  3 

cosmical,  51 

influence  on  chemical  theory,  41 

products  in  atmosphere,  51 

products  in  earth's  crust,  50 

products  in  soil  waters,  50 
Radium,  action  on  organic  matter, 
etc.,  33 

amount  in  pitchblende,  5 

discovery  of,  5 

emanation,  22 

energy  evolved  by,  34 

properties  of,  5,  32 
Rays,  alpha,  15,  16,  26 

beta,  15,  16 


INDEX 


gamma,  15,  16 
identification  of,  16,  25 
magnetic  deflection  of,  14 
photographing  track  of,  10 
types  of,  14 
Rubidium,  radio-activity  of,  3 

Spinthariscope,  13 

Stopping  power  of  substances,  39 

Thorium  X,  discovery  of,  18,  21 

Uranium  atom,  disintegration  of,  36 
Uranium  minerals,  radio-activity  of ,3 
Uranium  X,  discovery  of,  17,  21,  23 

X-ray  spectra,  44 
Zinc  sulphide  screen,  13 


• 


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26  1935 


22  Apr5  8  BF 


1947 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


